Electrolyte for lithium secondary battery and lithium secondary battery including the same

ABSTRACT

An electrolyte for a lithium secondary battery according to exemplary embodiments of the present inventing includes an organic solvent, a lithium salt, a first additive represented by a predetermined chemical formula, and a second additive represented by a predetermined chemical formula. A protective film is formed by the additives to suppress an expansion of a lithium secondary battery and improve storage property at high temperature.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to Korean Patent Application No.10-2020-0040945 filed Apr. 3, 2020, the disclosure of which is herebyincorporated by reference it its entirety.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to an electrolyte for a lithium secondarybattery and a lithium secondary battery including the same. Moreparticularly, the present invention relates to an electrolyte for alithium secondary battery including an organic solvent and a lithiumsecondary battery including the same.

2. Description of the Related Art

A secondary battery which can be charged and discharged repeatedly hasbeen widely employed as a power source of a mobile electronic devicesuch as a camcorder, a mobile phone, a laptop computer, etc., accordingto developments of information and display technologies. Recently, abattery pack including the secondary battery is being developed andapplied as an eco-friendly power source of an electric automobile suchas a hybrid vehicle.

The secondary battery includes, e.g., a lithium secondary battery, anickel-cadmium battery, a nickel-hydrogen battery, etc. The lithiumsecondary battery is highlighted due to high operational voltage andenergy density per unit weight, a high charging rate, a compactdimension, etc.

For example, the lithium secondary battery may include an electrodeassembly including a cathode, an anode and a separation layer(separator), and an electrolyte immersing the electrode assembly. Thelithium secondary battery may further include an outer case having,e.g., a pouch shape.

Recently, as an application of the lithium secondary battery isexpanded, the lithium secondary battery having higher capacity and powerhas been developed. For example, materials for a cathode and an anodecapable of providing higher capacity is being researched.

For example, a material that may replace a conventional carbon-basedmaterial is being researched. However, when the materials for thecathode and the anode are changed, an electrolyte contacting or reactingwith the cathode and the anode is also required to be modified or newlydesigned.

For example, Korean Published Patent Application No. 10-2012-0101499discloses a high voltage electrolyte for a lithium secondary battery.

SUMMARY OF THE INVENTION

According to an aspect of the present invention, there is provided anelectrolyte for a lithium secondary battery providing improved chemicalstability and operational reliability.

According to an aspect of the present invention, there is provided alithium secondary battery including the electrolyte and providingimproved chemical stability and operational reliability.

According to exemplary embodiments of the present invention, anelectrolyte for a lithium secondary battery includes an organic solvent,a lithium salt, a first additive represented by Chemical Formula 1 and asecond additive represented by Chemical Formula 2. Each amount of thefirst additive and the second additive is independently in a range from0.01 wt % to 5 wt % based on a total weight of the electrolyte:

In Chemical Formula 1, R¹ is a saturated hydrocarbon backbone structurehaving 3 to 10 carbon atoms, and n is an integer of 1 to 3.

In Chemical Formula 2, R¹¹ to R¹³ are each independently hydrogen, or asubstituted or unsubstituted alkyl group having 1 to 6 carbon atoms, andA is a substituent represented by Chemical Formula 3 or Chemical Formula4.

In Chemical Formulae 3 and 4, R¹⁴ and R¹⁵ are each independently asubstituted or unsubstituted alkyl group having 1 to 6 carbon atoms, asubstituted or unsubstituted aryl group having 6 to 12 carbon atoms, or—OR¹⁶.

R¹⁶ is a substituted or unsubstituted alkyl group having 1 to 6 carbonatoms, a substituted or unsubstituted aryl group having 6 to 12 carbonatoms, or an alkynyl group having 2 to 6 carbon atoms.

In some embodiments, in Chemical Formula 1, the saturated hydrocarbonbackbone structure of R¹ may be linear.

In some embodiments, the first additive may include at least one ofcompounds represented by Chemical Formulae 1-1 to 1-3.

In some embodiments, the second additive may include a compoundrepresented by Chemical Formula 4-1.

In some embodiments, an amount of the first additive may be in a rangefrom 0.1 wt % to 3 wt % based on the total weight of the electrolyte.

In some embodiments, an amount of the first additive may be in a rangefrom 0.1 wt % to 2 wt % based on the total weight of the electrolyte.

In some embodiments, an amount of the second additive may be in a rangefrom 0.1 wt % to 3 wt % based on the total weight of the electrolyte.

In some embodiments, a weight ratio of the first additive and the secondadditive may be from 1:0.25 to 1:3.

In some embodiments, the organic solvent may include at least oneselected from the group consisting of ethylene carbonate (EC), ethylmethyl carbonate (EMC), dimethyl carbonate (DMC), and diethyl carbonate(DEC).

In some embodiments, the lithium salt may include at least one oflithium hexafluorophosphate (LiPF₆) and lithium difluorophosphate(LiPO₂F₂).

In some embodiments, the electrolyte may further include a cycliccarbonate-based compound containing a double bond or afluorine-substituted cyclic carbonate-based compound.

In some embodiments, the electrolyte may further include a sultone-basedcompound.

In some embodiments, the electrolyte may further include a cyclicsulfonate-based compound.

According to exemplary embodiments, a lithium secondary battery includesa cathode, an anode, a separation layer interposed between the cathodeand the anode, and the electrolyte for a lithium secondary batteryaccording to embodiments as described above.

In the electrolyte for a lithium secondary battery according toembodiments of the present invention, a first additive and a secondadditive may suppress a side reaction between an electrode and theelectrolyte. For example, the first additive may passivate a metal of acathode, and the second additive may form an electrically and thermallystable protective film on a surface of an anode. In this case, the sidereaction between the electrode and the electrolyte may be suppressed, sothat depletion of the electrolyte may be prevented. Additionally, anexpansion of the battery due to a gas generation may be prevented.

Thus, life-span and capacity retention of the lithium secondary batterymay be improved, and reliability and storage property at hightemperature may be also improved.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic cross-sectional view illustrating a lithiumsecondary battery in accordance with exemplary embodiments.

DESCRIPTION OF THE INVENTION

According to exemplary embodiments of the present invention, anelectrolyte for a lithium secondary battery including an organicsolvent, a lithium salt and different first and second additives isprovided. The additives may form a protective film to suppress anexpansion of the lithium secondary battery and improve high-temperaturestorage properties. According to exemplary embodiments of the presentinvention, a lithium secondary battery including the electrolyte isprovided.

Hereinafter, the present invention will be described in detail withreference to the accompanying drawings. However, those skilled in theart will appreciate that such embodiments described with reference tothe accompanying drawings are provided to further understand the spiritof the present invention and do not limit subject matters to beprotected as disclosed in the detailed description and appended claims.

Electrolyte for Lithium Secondary Battery

An electrolyte for a lithium secondary battery (hereinafter, that may beabbreviated as an electrolyte) according to embodiments of the presentinvention may include an organic solvent, a lithium salt mixed ordissolved in the organic solvent and two or more types of additives. Forexample, the electrolyte may be used as a non-aqueous electrolyte for alithium secondary battery.

The organic solvent may include an organic compound that may provide asufficient solubility to the lithium salt and the additives, and may nothave a reactivity with the lithium secondary battery. In exemplaryembodiments, the organic solvent may include a carbonate-based solvent,an ester-based solvent, an ether-based solvent, a ketone-based solvent,an alcohol-based solvent, an aprotic solvent, or the like. These may beused alone or in combination thereof.

Examples of the carbonate-based solvent may include dimethyl carbonate(DMC), ethyl methyl carbonate (EMC), methyl propyl carbonate, ethylpropyl carbonate, diethyl carbonate (DEC), dipropyl carbonate, propylenecarbonate (PC), ethylene carbonate (EC), fluoroethylene carbonate (FEC),butylene carbonate, etc.

Examples of the ester-based solvent may include methyl acetate (MA),ethyl acetate (EA), n-propyl acetate (n-PA), 1,1-dimethylethyl acetate(DMEA), methyl propionate (MP), ethyl propionate (EP),gamma-butyrolacton (GBL), decanolide, valerolactone, mevalonolactone,caprolactone, etc.

Examples of the ether-based organic solvent may include dibutyl ether,tetraethylene glycol dimethyl ether (TEGDME), diethylene glycol dimethylether (DEGDME), dimethoxy ethane, 2-methyltetrahydrofuran,tetrahydrofuran, etc.

Cyclohexanone may be used as the ketone-based solvent. Examples of thealcohol-based solvent may include ethyl alcohol, isopropyl alcohol, etc.

The aprotic solvent may include a nitrile-based solvent, an amide-basedsolvent such as dimethyl formamide (DMF), a dioxolane-based solvent suchas 1,3-dioxolane, a sulfolane-based solvent, etc.

In a preferable embodiment, the carbonate-based solvent may be used asthe organic solvent. For example, the organic solvent may includeethylene carbonate (EC), ethyl methyl carbonate (EMC), dimethylcarbonate (DMC), diethyl carbonate (DEC), or a combination thereof.Preferably, a mixture of EC and EMC may be used as the organic solvent.

The lithium salt may include, e.g., a compound represented by Li⁺X⁻.

Non-limiting examples of the anion (X—) of the lithium salt may includeF⁻, Cl⁻, Br⁻, I⁻, NO₃ ⁻, N(CN)₂ ⁻, BF₄ ⁻, ClO₄ ⁻, PF₆ ⁻, SbF₆ ⁻, AsF₆ ⁻,(CF₃)₂PF₄ ⁻, (CF₃)₃PF₃ ⁻, (CF₃)₄PF₂ ⁻, (CF₃)₅PF⁻, (CF₃)₆P⁻, CF₃SO₃ ⁻,CF₃CF₂SO₃ ⁻, (CF₃SO₂)₂N⁻, (FSO₂)₂N⁻, CF₃CF₂(CF₃)₂CO⁻, (CF₃SO₂)₂CH⁻,(SF₅)₃C⁻, (CF₃SO₂)₃C⁻, CF₃(CF₂)₇SO₃ ⁻, CF₃CO₂ ⁻, CH₃CO₂ ⁻, SCN⁻,(CF₃CF₂SO₂)₂N⁻, PO₂F₂ ⁻, etc. These may be used alone or in acombination thereof.

Preferably, a mixture of lithium hexafluorophosphate (LiPF₆) and lithiumdifluorophosphate (LiPO₂F₂) may be used as the lithium salt. Forexample, LiPO₂F₂ may form a film having improved thermal stability on anelectrode surface. LiPF₆ and LiPO₂F₂ may be mixed in a weight ratio of1:0.5 to 1:2. Within the weight ratio range, the electrolyte may haveenhanced ionic conductivity and electrode protection properties.

In an embodiment, the lithium salt may be included in a concentrationfrom about 0.01 M to about 5 M, preferably from about 0.01 M to 2 M withrespect to the organic solvent. Within the above range, a transfer oflithium ions and/or electrons may be promoted during charging anddischarging of the lithium secondary battery, thereby providing improvedcapacity.

The first additive may be represented by the following Chemical Formula(1).

In the Chemical Formula 1 above, R¹ is a saturated hydrocarbon backbonestructure having 3 to 10 carbon atoms, and n is an integer of 1 to 3.

The term “saturated hydrocarbon backbone structure” used herein mayindicate a substituent derived from a hydrocarbon that does not containan unsaturated bond. For example, the saturated hydrocarbon backbonestructure may indicate a radical from which one or more hydrogen isremoved from a saturated hydrocarbon, and may include an alkyl group andan alkylene group. The saturated hydrocarbon backbone structure may belinear, branched or cyclic.

Preferably, the saturated hydrocarbon backbone structure may be linear,so that a coordination bond with a metal element contained in a cathodeor a chelating with the metal element may be effectively formed.

For example, as a lithium secondary battery is used, a metal (e.g., atransition metal) may be eluted from the cathode. The eluted metal maybe electrodeposited on an anode, thereby deteriorating a performance ofthe anode. Additionally, when the lithium secondary battery is driven ata high voltage, a coating on the surface of the cathode may bedecomposed to cause a side reaction between the surface of the cathodeand the electrolyte.

The first additive may stabilize a structure of the cathode bycoordinating with the metal of the cathode. In this case, when thelithium secondary battery is used and stored at a high temperature, theelution of the metal, a gas generation and an expansion of volume(thickness) may be suppressed. Accordingly, life-span andhigh-temperature storage properties of the lithium secondary battery maybe improved. Further, an increase of a resistance of the battery may besuppressed when being driven at a high voltage.

In exemplary embodiments, in Chemical Formula 1, when n is 1, R¹ may bean alkyl group having 3 to 10 carbon atoms. In this case, the firstadditive may include a compound represented by Chemical Formula 1-1below.

In exemplary embodiments, when n is 2, R¹ may be an alkylene grouphaving 3 to 10 carbon atoms. In this case, the first additive mayinclude a compound represented by Chemical Formula 1-2 below.

In exemplary embodiments, when n is 3, R¹ may be an alkylene grouphaving 3 to 10 carbon atoms. In the alkylene group, a terminal end ofthe hydrocarbon backbone structure may be substituted with —OPF₂.Additionally, at least one —OPF₂ group may be bonded at an intermediatecarbon atom of the hydrocarbon backbone structure. For example, when R¹is a butylene group in which a secondary carbon atom is substituted with—OPF₂, the first additive may include a compound represented by ChemicalFormula 1-3 below.

The first additive may be included in an amount from 0.01 to 5 weightpercent (wt %) based on a total weight of the electrolyte. If the amountof the first additive is less than 0.01 wt %, the stabilizing effect ofthe cathode structure may be insufficient. If the amount of the firstadditive exceeds 5 wt %, an internal resistance of the lithium secondarybattery may be excessively increased or the capacity of the lithiumsecondary battery may be decreased.

Preferably, the first additive may be included in an amount from 0.1 wt% to 2 wt %, more preferably from 0.3 wt % to 2 wt % or from 0.5 wt % to2 wt %.

The second additive may be represented by Chemical Formula 2 below.

In the Chemical Formula 2 above, R¹¹ to R¹³ are each independentlyhydrogen or a substituted or unsubstituted alkyl group having 1 to 6carbon atoms.

A is a substituent represented by Chemical Formula 3 or Chemical Formula4 below.

In the Chemical Formulae 3 and 4, R¹⁴ and R¹⁵ are each independently asubstituted or unsubstituted alkyl group having 1 to 6 carbon atoms, asubstituted or unsubstituted aryl group having 6 to 12 carbon atoms, or—OR¹⁶.

R¹⁶ is a substituted or unsubstituted alkyl group having 1 to 6 carbonatoms, a substituted or unsubstituted aryl group having 6 to 12 carbonatoms, or an alkynyl group having 2 to 6 carbon atoms.

For example, the second additive may include a compound represented byFormula 4-1 below.

The second additive may form a coating or a film having excellentthermal stability on the electrode. For example, the second additive mayform a complex with the eluted metal when the lithium secondary batteryis operated. Accordingly, the eluted metal component may be removed fromthe electrolyte, so that side reactions caused by the metal component(e.g., a formation of dendrite of the eluted metal) may be suppressed.Additionally, the complex may be provided as the coating or the film toimprove stability of the electrode.

The second additive may be included in an amount from 0.01 wt % to 5 wt% based on the total weight of the electrolyte. If the amount of thesecond additive is less than 0.01 wt %, the stabilizing effect of theanode structure may be insufficient. If the amount of the secondadditive exceeds 5 wt %, the internal resistance of the lithiumsecondary battery may be excessively increased or the capacity may bedecreased.

Preferably, the second additive may be included in an amount from 0.1 wt% to 3 wt %, more preferably 0.3 wt % to 2 wt % or 0.5 wt % to 2 wt %.

In exemplary embodiments, a sum of the amounts of the first additive andthe second additive may be from 0.5 wt % to 10 wt % based on the totalweight of the electrolyte. Within the above-described range, the cathodeand the anode of the battery may be effectively protected. Preferably,the sum of the amounts of the first additive and the second additive maybe from 1 wt % to 10 wt %, or from 1 wt % to 5 wt % based on the totalweight of the electrolyte.

In exemplary embodiments, the first additive and the second additive maybe included in a weight ratio from 1:0.25 to 1:3. Within the weightratio range, a cathode protection from the first additive and an anodeprotection from the second additive may be balanced. If the amount ofthe first additive is less than the above range, deterioration of thecathode may be accelerated relatively to the anode. If the amount thefirst additive exceeds the above range, deterioration of the anode maybe accelerated relatively to the cathode.

Preferably, the weight ratio of the first additive and the secondadditive may be from 1:0.5 to 1:2, more preferably, from 1:0.5 to 1:1.

In exemplary embodiments, the electrolyte may include an additionaladditive such as a cyclic carbonate-based compound containing a doublebond, a fluorine-substituted cyclic carbonate-based compound, asultone-based compound, a cyclic sulfonate-based compound, or the like.

The cyclic carbonate-based compound containing the double bond mayinclude vinylene carbonate, vinyl ethylene carbonate, or the like.

The fluorine-substituted cyclic carbonate-based compound may includefluoroethylene carbonate.

The cyclic carbonate-based compound containing the double bond and thefluorine-substituted cyclic carbonate-based compound may improve thermaland electrical durability of the film formed on the electrode surface.

For example, each of the cyclic carbonate-based compound containing thedouble bond and the fluorine-substituted cyclic carbonate-based compoundmay be included in an amount from 0.1 wt % to 5 wt % based on the totalweight of the electrolyte. If the amount is less than 0.1 wt %, thedurability of the film may be degraded. If the amount is more than 5 wt% by weight, a thickness of the film may be excessively increased. Inthis case, the resistance of the battery may increase to degrade a powerof the battery.

The sultone-based compound may include 1,3-propane sultone, 1,3-propenesultone, 1,4-butane sultone, or the like.

The cyclic sulfonate-based compound may include 1,2-ethylene sulfate,1,2-propylene sulfate, or the like.

The sultone-based compound and the cyclic sulfonate-based compound mayform a more stable ion conductive film on the electrode surface. Forexample, the sultone-based compound and the cyclic sulfonate-basedcompound may form a structurally stable protective film on the electrodesurface through a reaction with the second additive.

For example, each of the sultone-based compound and the cyclicsulfonate-based compound may be included in an amount from 0.1 wt % to 5wt % based on the total weight of the electrolyte. If the amount is lessthan 0.1 wt %, the durability of the film may be degraded. If the amountis more than 5 wt %, the thickness of the film may be excessivelyincreased. In this case, the resistance of the battery may increase todegrade the power of the battery.

Lithium Secondary Battery

FIG. 1 is a schematic cross-sectional view illustrating a lithiumsecondary battery in accordance with exemplary embodiments.

Referring to FIG. 1 , a lithium secondary battery 100 may include anelectrode assembly including a cathode 130, an anode 140 and aseparation layer 150 interposed between the cathode and the anode. Theelectrode assembly may be accommodated in a case 170 together with theelectrolyte according to the above-described exemplary embodiments to beimpregnated therein.

The cathode 130 may include a cathode active material layer 115 formedby coating a cathode active material on a cathode current collector 110.The cathode active material may include a compound capable of reversiblyintercalating and deintercalating lithium ions.

In exemplary embodiments, the cathode active material may include alithium-transition metal oxide. For example, the lithium-transitionmetal oxide may include nickel (Ni), and may further include at leastone of cobalt (Co) and manganese (Mn).

For example, the lithium-transition metal oxide may be represented byChemical Formula 5 below.Li_(1+a)Ni_(1-(x+y))Co_(x)M_(y)O₂  [Chemical Formula 5]

In the Chemical Formula 5 above, −0.05≤a≤0.15, 0.01≤x≤0.3, 0.01≤y≤0.3,and M may include at least one element selected from Mn, Mg, Sr, Ba, B,Al, Si, Ti, Zr and W.

A slurry may be prepared by mixing and stirring the cathode activematerial in a solvent with a binder, a conductive agent and/or adispersive agent. The slurry may be coated on the cathode currentcollector 110, and then dried and pressed to form the cathode 130.

The cathode current collector 110 may include stainless-steel, nickel,aluminum, titanium, copper or an alloy thereof. Preferably, aluminum oran alloy thereof may be used.

The binder may include an organic based binder such as a polyvinylidenefluoride-hexafluoropropylene copolymer (PVDF-co-HFP),polyvinylidenefluoride (PVDF), polyacrylonitrile,polymethylmethacrylate, etc., or an aqueous based binder such asstyrene-butadiene rubber (SBR) that may be used with a thickener such ascarboxymethyl cellulose (CMC).

For example, a PVDF-based binder may be used as a cathode binder. Inthis case, an amount of the binder for forming the cathode activematerial layer may be reduced, and an amount of the cathode activematerial may be relatively increased. Thus, capacity and power of thelithium secondary battery may be further improved.

The conductive agent may be added to facilitate electron mobilitybetween active material particles. For example, the conductive agent mayinclude a carbon-based material such as graphite, carbon black,graphene, carbon nanotube, etc., and/or a metal-based material such astin, tin oxide, titanium oxide, a perovskite material such as LaSrCoO₃or LaSrMnO₃, etc.

The anode 140 may include an anode current collector 120 and an anodeactive material layer 125 formed by coating an anode active material onthe anode current collector 120.

In exemplary embodiments, a silicon (Si)-based compound may be used asthe anode active material. In some embodiments, silicon carbide (SiC) ora silicon-carbon particle including a carbon core and a silicon coatinglayer may be used as the anode active material.

The silicon-carbon particle may be formed by, e.g., depositing a siliconlayer on a surface of a graphite core. In an embodiment, thesilicon-carbon particle may be formed by coating the silicon layer on acommercially available graphite particle through a chemical vapordeposition (CVD) process using a silicon precursor compound such as asilane-based compound.

In some embodiments, the silicon-carbon particle may have a structure inwhich a plurality of carbon coating layers and silicon coating layersare alternately coated or stacked on the graphite core.

The anode current collector 120 may include gold, stainless-steel,nickel, aluminum, titanium, copper or an alloy thereof, preferably, mayinclude copper or a copper alloy.

In some embodiments, the anode active material may be mixed and stirredtogether with a binder, a conductive agent and/or a dispersive additivein a solvent to form a slurry. The slurry may be coated on the anodecurrent collector 120, and dried and pressed to obtain the anode 140.The conductive agent substantially the same as or similar to that asmentioned above may be used.

In some embodiments, the binder for the anode may includestyrene-butadiene rubber (SBR) that may be reacted with the additives ofthe electrolyte as described above. In some embodiments, a thickenersuch as carboxymethyl cellulose (CMC) may be used together with SBR.

The separation layer 150 may be interposed between the cathode 130 andthe anode 140. The separation layer 150 may include a porous polymerfilm prepared from, e.g., a polyolefin-based polymer such as an ethylenehomopolymer, a propylene homopolymer, an ethylene/butene copolymer, anethylene/hexene copolymer, an ethylene/methacrylate copolymer, or thelike. The separation layer may also include a non-woven fabric formedfrom a glass fiber with a high melting point, a polyethyleneterephthalate fiber, or the like.

In some embodiments, an area and/or a volume of the anode 140 (e.g., acontact area with the separation layer 150) may be greater than that ofthe cathode 130. Thus, lithium ions generated from the cathode 130 maybe easily transferred to the anode 140 without a loss by, e.g.,precipitation or sedimentation.

In exemplary embodiments, an electrode cell 160 may be defined by thecathode 130, the anode 140 and the separation layer 150, and a pluralityof the electrode cells 160 may be stacked to form the electrode assemblyhaving, e.g., a jelly roll shape. For example, the electrode assemblymay be formed by winding, laminating or folding of the separation layer.

The electrode assembly may be accommodated in the case 170 together withthe electrolyte according to exemplary embodiments to form the lithiumsecondary battery.

An electrode tab may be formed from each of the cathode currentcollector 110 and the anode current collector 120 to extend to one endof the case 170. The electrode tabs may be welded together with the oneend of the case 170 to form electrode leads exposed at an outside of thecase 170.

The lithium secondary battery may be fabricated into a cylindrical shapeusing a can, a prismatic shape, a pouch shape, a coin shape, etc.

Hereinafter, preferred embodiments are proposed to more concretelydescribe the present invention. However, the following examples are onlygiven for illustrating the present invention and those skilled in therelated art will obviously understand that various alterations andmodifications are possible within the scope and spirit of the presentinvention. Such alterations and modifications are duly included in theappended claims.

EXAMPLES AND COMPARATIVE EXAMPLES

(1) A slurry was prepared by mixing Li[Ni_(0.8)Co_(0.1)Mn_(0.1)]O₂ as acathode active material, carbon black as a conductive material andpolyvinylidene fluoride (PVdF) as a binder in a weight ratio of 96:2:2.The slurry was uniformly applied to an aluminum foil having a thicknessof 15 μm, and vacuum-dried at 130° C. The dried slurry was pressed toprepare a cathode for a lithium secondary battery having a density of3.667 g/cm³.

(2) A slurry including 97 wt % of an anode active material in whichartificial graphite and natural graphite were mixed in a weight ratio of7:3, 1 wt % of styrene-butadiene rubber (SBR) as a binder and 2 wt % ofcarboxymethyl cellulose (CMC) as a thickener was prepared. The anodeslurry was uniformly coated, dried and pressed on a 15 μm-thick copperfoil to prepare an anode having a density of 1.684 g/cm³.

(3) After dissolving 1 M LiPF₆ in a mixed solvent of EC/EMC (1:3; volumeratio), vinylene carbonate 0.5 wt %, fluoroethylene carbonate 1 wt %,LiPO₂F₂ 1 wt %, 1,3-propane sultone 0.5 wt %, 1,3-propene sultone 1 wt %and 1,2-ethylene sulfonate 0.5 wt % were mixed to prepare a baseelectrolyte.

The electrolytes of Examples and Comparative Examples were prepared byadding the additives shown in Table 1 to the base electrolyte.

(4) The cathode and the anode obtained as described above were notchedwith a proper size and stacked, and a separator (polyethylene,thickness: 20 μm) was interposed between the cathode and the anode toform an electrode cell. Each tab portion of the cathode and the anodewas welded. The welded cathode/separator/anode assembly was inserted ina pouch, and three sides of the pouch except for an electrolyteinjection side were sealed. The tab portions were also included insealed portions. The electrolyte according to each Examples andComparative Examples was injected through the electrolyte injectionside, and then the electrolyte injection side was also sealed.Subsequently, the above structure was impregnated for more than 12 hoursto prepare a lithium secondary battery with a capacity grade of about 2Ah.

TABLE 1 First Additive Second Additive Amount Amount (wt % based on (wt% based on a total weight a total weight Weight Ratio of the of the ofthe first additive and Type electrolyte) Type electrolyte) the secondadditive Example 1 Chemical 1.0 Chemical 1.0 1:1  Formula 1-1 Formula4-1 Example 2 Chemical 1.0 Chemical 1.0 1:1  Formula 1-2 Formula 4-1Example 3 Chemical 1.0 Chemical 1.0 1:1  Formula 1-3 Formula 4-1 Example4 Chemical 0.1 Chemical 0.1 1:1  Formula 1-1 Formula 4-1 Example 5Chemical 0.1 Chemical 1.0 1:10 Formula 1-1 Formula 4-1 Example 6Chemical 2.0 Chemical 1.0 2:1  Formula 1-1 Formula 4-1 Example 7Chemical 1.0 Chemical 0.1 10:1   Formula 1-1 Formula 4-1 Example 8Chemical 1.0 Chemical 2.0 1:2  Formula 1-1 Formula 4-1 Example 9Chemical 2.0 Chemical 2.0 1:1  Formula 1-1 Formula 4-1 Example 10Chemical 5.0 Chemical 5.0 1:1  Formula 1-1 Formula 4-1 Comparative —Example 1 Comparative Chemical 1.0 — Example 2 Formula 1-1 ComparativeChemical 1.0 Chemical 6.0 1:6  Example 3 Formula 1-1 Formula 4-1Comparative — Chemical 1.0 — Example 4 Formula 4-1 Comparative Chemical6.0 Chemical 1.0 6:1  Example 5 Formula 1-1 Formula 4-1

Experimental Example 1: Evaluation on Initial Capacity and Resistance

Charging (CC/CV ⅓C 4.2V 0.05 C CUT-OFF) and discharging (CC ⅓ C 2.5VCUT-OFF) of the electrolyte-injected secondary batteries of Examples andComparative Examples were performed, and an initial discharge capacityof each battery was measured.

At a point where an SOC (State of Charge) of the battery was set to 60%,the C-rate sequentially increased or decreased to 0.2 C, 0.5 C, 1.0 C,1.5 C, 2.0 C, 2.5 C and 3.0 C, and terminal points of a voltage were setas a linear equation when charging and discharging of the correspondingC-rate proceeded for 10 seconds. A slope of the linear equation wasadopted as DCIR (direct current internal resistance).

Experimental Example 2: Evaluation of Gas Generation after Storage atHigh Temperature for 2 Weeks

The secondary batteries injected with the electrolytes of Examples andComparative Examples were left at 60° C. for 2 weeks, and then left atroom temperature for 30 minutes. The batteries were placed in a chamberfor measuring an amount of a gas generation. After forming a vacuum inthe chamber, nitrogen gas was filled to form an atmospheric pressure,and a nitrogen volume (V₀) and a chamber internal pressure (P₀) weremeasured. After forming a vacuum at an inside of the chamber again, ahole was made in the battery, and a pressure at the inside of thechamber (P₁) was measured. The amount of the gas generation wascalculated according to the following equation.Gas generation amount (mL)=(V ₀ /P ₀)*P ₁

Experimental Example 3: Evaluation of Capacity and Resistance after 8Weeks of Storage at High Temperature

The secondary batteries injected with the electrolytes of Examples andComparative Examples were stored for 8 weeks in a chamber at 60° C., andleft at room temperature for 30 minutes.

Thereafter, DCIR of each battery was measured again by the method asdescribed in Experimental Example 1.

After the DCIR measurement, a capacity measured when 1C rate CCdischarge (2.7V cut-off) was performed was denominated by the initialdischarge capacity to be represented as a percentage.

The evaluation results are shown in Table 2 below.

TABLE 2 Gas Generation Properties after High Temperature after HighStorage for 8 weeks Temperature Increased Initial Property Storage forCapacity Ratio of Capacity DCIR 2 weeks Capacity Retention DCIR DCIR(mAh) (mΩ) (ml) (mAh) (%) (mΩ) (%) Example 1 2044 39.91 8.1 1739 8557.97 145 Example 2 2035 41.21 9.3 1711 84 60.27 146 Example 3 202342.11 10.3 1680 83 62.17 148 Example 4 2066 41.14 15.2 1570 76 65.11 158Example 5 2023 43.81 13.4 1618 80 67.47 154 Example 6 2020 42.71 11.01686 83 63.47 149 Example 7 2030 42.56 14.3 1604 79 65.12 153 Example 82011 44.61 10.7 1669 83 66.02 148 Example 9 2016 45.51 10.5 1653 8267.81 149 Example 10 2031 42.03 12.4 1645 81 63.47 151 Comparative 206941.34 20.2 1345 65 69.45 168 Example 1 Comparative 2056 37.5 17.5 148072 65.25 174 Example 2 Comparative 2011 46.91 16.7 1488 74 78.81 168Example 3 Comparative 2053 43.91 18.3 1458 71 72.45 165 Example 4Comparative 2018 45.31 16.9 1453 72 75.67 167 Example 5

Referring to Table 2, in the batteries of Examples where the firstadditive and the second additive were used, the volume expansion and theincrease of the internal resistance were suppressed while increasing thecapacity retention relatively to those in the batteries of ComparativeExamples.

What is claimed is:
 1. An electrolyte for a lithium secondary battery,comprising: an organic solvent; a lithium salt; a first additiverepresented by Chemical Formula 1; and a second additive represented byChemical Formula 2, wherein each amount of the first additive and thesecond additive is independently in a range from 0.01 wt % to 5 wt %based on a total weight of the electrolyte:

wherein, in Chemical Formula 1, R¹ is a saturated hydrocarbon backbonestructure having 3 to 10 carbon atoms, and n is an integer of 1 to 3,

wherein, in Chemical Formula 2, R¹¹ to R¹³ are each independentlyhydrogen, or a substituted or unsubstituted alkyl group having 1 to 6carbon atoms, and A is a substituent represented by Chemical Formula 3or Chemical Formula 4,

wherein, in Chemical Formulae 3 and 4, R¹⁴ and R¹⁵ are eachindependently a substituted or unsubstituted alkyl group having 1 to 6carbon atoms, a substituted or unsubstituted aryl group having 6 to 12carbon atoms, or —OR¹⁶, and R¹⁶ is a substituted or unsubstituted alkylgroup having 1 to 6 carbon atoms, a substituted or unsubstituted arylgroup having 6 to 12 carbon atoms, or an alkynyl group having 2 to 6carbon atoms.
 2. The electrolyte for a lithium secondary batteryaccording to claim 1, wherein, in Chemical Formula 1, the saturatedhydrocarbon backbone structure of R¹ is linear.
 3. The electrolyte for alithium secondary battery according to claim 1, wherein the firstadditive includes at least one of compounds represented by ChemicalFormulae 1-1 to 1-3:


4. The electrolyte for a lithium secondary battery according to claim 1,wherein the second additive includes a compound represented by ChemicalFormula 4-1:


5. The electrolyte for a lithium secondary battery according to claim 1,wherein an amount of the first additive is in a range from 0.1 wt % to 3wt % based on the total weight of the electrolyte.
 6. The electrolytefor a lithium secondary battery according to claim 1, wherein an amountof the first additive is in a range from 0.1 wt % to 2 wt % based on thetotal weight of the electrolyte.
 7. The electrolyte for a lithiumsecondary battery according to claim 1, wherein an amount of the secondadditive is in a range from 0.1 wt % to 3 wt % based on the total weightof the electrolyte.
 8. The electrolyte for a lithium secondary batteryaccording to claim 1, wherein a weight ratio of the first additive andthe second additive is from 1:0.25 to 1:3.
 9. The electrolyte for alithium secondary battery according to claim 1, wherein the organicsolvent includes at least one selected from the group consisting ofethylene carbonate (EC), ethyl methyl carbonate (EMC), dimethylcarbonate (DMC), and diethyl carbonate (DEC).
 10. The electrolyte for alithium secondary battery according to claim 1, wherein the lithium saltincludes at least one of lithium hexafluorophosphate (LiPF₆) and lithiumdifluorophosphate (LiPO₂F₂).
 11. The electrolyte for a lithium secondarybattery according to claim 1, further comprising a cycliccarbonate-based compound containing a double bond or afluorine-substituted cyclic carbonate-based compound.
 12. Theelectrolyte for a lithium secondary battery according to claim 1,further comprising a sultone-based compound.
 13. The electrolyte for alithium secondary battery according to claim 1, further comprising acyclic sulfonate-based compound.
 14. A lithium secondary battery,comprising: a cathode; an anode; a separation layer interposed betweenthe cathode and the anode; and the electrolyte for a lithium secondarybattery according to claim 1.